Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive. In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE. The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (Rel. 8 LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP specification TR 25.913, “Requirements for Evolved UTRA and Evolved UTRAN”, ver. 9.0.0, freely available at www/3gpp.org.
In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in Rel. 8 LTE.
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN comprises eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNB hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state UEs, the SGW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called sub-frames. In 3GPP LTE each sub-frame is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each sub-frame consists of a given number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each of OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consist of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 3.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 3. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available free of charge at http://www.3gpp.org and incorporated herein by reference). The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Cell search procedures are the first set of tasks performed by a mobile device in a cellular system after initial power-up. Only after the search and registration procedures, a mobile device is able to receive and initiate voice and data calls. A typical cell search procedure in LTE may involve a combination of carrier frequency determination, timing synchronization and identification of unique cell identifier. These procedures are typically facilitated by specific synchronization signals transmitted by the base station (BTS). However, these synchronization signals are not continuously used in connected modes for a mobile device. Hence, only minimum resources in terms of power, subcarrier allocation and time slice are allocated for synchronization signals.
The cell search procedure enables the UE to determine the time and frequency parameters which are necessary to demodulate the downlink and to transmit uplink signals with the correct timing. The first phase of the cell search includes an initial synchronization. Accordingly, the UE detects an LTE cell and decodes all the information required for registering to the detected cell. The procedure makes use of two physical signals which are broadcast in the central 62 subcarriers of each cell, the primary and secondary synchronization signals (PSS and SSS, respectively). These signals enable time and frequency synchronization. Their successful detection provides a UE with the physical cell-ID, cyclic prefix length, and information as to whether FDD or TDD is employed. In particular, in LTE, when a terminal is switched on, it detects the primary synchronization signal, which for FDD is transmitted in the last OFDM symbol of the first time slot of the first subframe (subframe 0) in a radio frame (for TDD the location is slightly different, but still well-determined). This enables the terminal to acquire the slot boundary independently of the chosen cyclic prefix selected for the cell. After the mobile terminal has found the 5 millisecond timing (slot boundaries), the secondary synchronization signal is looked for. Both the PSS and SSS are transmitted on 62 of the 72 reserved subcarriers around the DC carrier. In the next step, the UE shall detect a physical broadcast channel (PBCH) which, similarly to the PSS and SSS is mapped only to the central 72 subcarriers of a cell. The PBCH contains the Master Information Block (MIB) including information about the system resources. In LTE up to Release 10, MIB had a length of 24 bits (14 bits of which are currently used and 10 bits are spare). MIB includes information concerning the downlink system bandwidth, physical HARQ Indicator Channel (PHICH) structure, and 8 most significant bits of the System Frame Number (SFN).
After successful detection of the master information block (MIB) which includes a limited number of the most frequently transmitted parameters essential for initial access to the cell, the terminal activates the system bandwidth, meaning that it has to be able to receive and detect signals across the indicated downlink system bandwidth. After acquiring the downlink system bandwidth, the UE may proceed with receiving further required system information on the so-called System Information Blocks (SIB). In LTE Release 10, SIB Type 1 to SIB Type 13 are defined, carrying different information elements required for certain operations. For instance, in case of FDD the SIB Type 2 (SIB2) includes the UL carrier frequency and the UL bandwidth. The various SIBs are transmitted on a Physical Downlink Shared Channel (PDSCH) and thus (cf. details to PDSCH and PDCCH below) the respective allocations are assigned by a Physical Downlink Control Channel (PDCCH). Before the terminal (UE) is able to correctly detect such (or any) PDCCH, it needs to know the downlink system bandwidth from the MIB.
The above mentioned Cell identity (cell-ID) will identify the cell uniquely within the PLMN. The cell identity is a global cell-ID that is used to identify the cell from an Operation and Maintenance (OAM) perspective. It is transmitted in the System Information and is designed for eNodeB management within the core network. The global cell identity is also used for UE to identify a specific cell in terms of RRC/NAS layer processing. Physical cell identity is the cell identity at physical layer. The physical cell identity has a range of 0 to 503 and it is used to scramble the data to help the user equipment separate information from the different transmitters. A physical cell ID will determine the primary and secondary synchronization signal sequence. It is similar to the Scrambling Codes from UMTS. There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity NIDcell=3NID(1)+NID(2) is thus uniquely defined by a number NID(1) in the range of 0 to 167, representing the physical-layer cell-identity group, and a number NID(2) in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group.
Synchronization signal is composed of a primary synchronization signal (PSS) and secondary synchronization signal (SSS). The sequence used for the primary synchronization signal is generated from a frequency-domain Zadoff-Chu sequence according to NID(2). By detecting primary synchronization signal, NID(2) could be detected. The sequence used for the second synchronization signal is an interleaved concatenation of two binary sequences with length of 31 bits. The concatenated sequence is scrambled with a scrambling sequence given by the primary synchronization signal. The SSS sequences are based on maximum length sequences, known as M-sequences, which can be created b cycling through every possible state of a shift register of length n. This results in a sequence of length 2n-1. In particular, the two 31-bit long binary sequences to be concatenated are such M-sequences. For further details on the primary and secondary synchronization signal, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 12)”, version 12.1.0, section 6.11, available free of charge at http://www.3gpp.org and incorporated herein by reference.
After receiving the PPS and SSS, the timing is adapted by the receiving UE. In particular, the UE synchronizes its receiver to the downlink transmission received from the synchronization source (eNB). Then, the uplink timing is adjusted. This is performed by applying a time advance at the UE transmitter, relative to the received downlink timing in order to compensate for propagation delays varying for different UEs. The timing advance procedure is described concisely in Section 18.2.2 of the book “LTE The UMTS Long Term Evolution: From theory to practice”, 2nd edition, ed. By S. Sesia, I. Toufik, M. Baker, Wiley, 2011.
Extension of LTE operation on unlicensed spectrum is currently being considered in 3GPP as a possible solution for increasing user and overall cell throughput. The reason for extending LTE to unlicensed bands is the ever-growing demand for wireless broadband data in conjunction with the limited amount of licensed bands. Unlicensed spectrum therefore is more and more considered by cellular operators as a complementary tool augment their service offering. The advantage of LTE in unlicensed bands compared to relying on other radio access technologies (RATs) such as Wi-Fi is that complementing the LTE platform with unlicensed spectrum access enables operators and vendors to leverage the existing or planned investments in LTE/EPC hardware in the radio and core network.
However, it has to be taken into account that unlicensed spectrum access can never match the qualities of licensed spectrum due to the inevitable coexistence with other radio access technologies (RATS) in the unlicensed spectrum. LTE operation on unlicensed bands will therefore at least in the beginning be considered rather a complement to LTE on licensed spectrum than stand-alone operation on unlicensed spectrum. Therefore, solutions being currently developed envisage LTE operation on unlicensed bands in conjunction with at least one licensed band. This scheme is indicated as Licensed Assisted Access (LAA). Future stand-alone operation of LTE on unlicensed spectrum without relying on LAA is however not excluded.
The current intended general LAA approach at 3GPP is to make use of the already specified Rel-12 carrier aggregation (CA) framework as much as possible where the CA framework configuration comprises a so-called primary cell (PCell) carrier and one or more secondary cell (SCell) carriers. CA supports in general both self-scheduling of cells (scheduling information and user data are transmitted on the same carrier) and cross-carrier scheduling between cells, while scheduling information in terms of PDCCH/EPDCCH and user data in terms of PDSCH/PUSCH are transmitted on different carriers.
The basic envisioned approach at 3GPP is that the PCell will be operated on a licensed band, while one or more SCells will be operated on unlicensed bands. The benefit of this strategy is that the PCell can be used for reliable transmission of control messages and user data with high quality of service (QoS) demands, such as for example voice and video, while a PCell on unlicensed spectrum might yield, depending on the scenario, to some extent significant QoS reduction due to inevitable coexistence with other RATs.
It has been agreed during RAN1#78bis, that the LAA investigation at 3GPP will focus on unlicensed bands at 5 GHz (this is described in 3GPP RAN1#78bis Chairman Notes, October 2014). One of the most critical issues is therefore the coexistence with Wi-Fi (see as reference IEEE 802.11 specification) systems operating at these unlicensed bands. In order to support fair coexistence between LTE and other technologies such as Wi-Fi as well as fairness between different LTE operators in the same unlicensed band, the channel access of LTE for unlicensed bands has to abide by certain sets of regulatory rules which depend on region and considered frequency band. A comprehensive description of the regulatory requirements for operation on unlicensed bands at 5 GHz is given in R1-144348, “Regulatory Requirements for Unlicensed Spectrum”, Alcatel-Lucent et al., RAN1#78bis, September 2014, which is herein enclosed by reference. Depending on region and band, regulatory requirements that have to be taken into account when designing LAA procedures comprise Dynamic Frequency Selection (DFS), Transmit Power Control (TPC), Listen Before Talk (LBT) and discontinuous transmission with limited maximum transmission duration. The intention of the 3GPP is to target a single global framework for LAA which basically means that all requirements for different regions and bands at 5 GHz have to be taken into account for the system design.
DFS is required for certain regions and bands in order to detect interference from radar systems and to avoid co-channel operation with these systems. The intention is furthermore to achieve a near-uniform loading of the spectrum. The DFS operation and corresponding requirements are associated with a master-slave principle. The master shall detect radar interference, can however rely on another device, that is associated with the master, to implement the radar detection.
The operation on unlicensed bands at 5 GHz is in most regions limited to rather low transmit power levels compared to the operation on licensed bands resulting in small coverage areas. A further requirement for certain regions and bands is the use of TPC in order to reduce the average level of interference caused to other devices operating on the same unlicensed band.
Following the European regulation regarding LBT, devices have to perform a Clear Channel Assessment (CCA) before occupying the radio channel. It is only allowed to initiate a transmission on the unlicensed channel after detecting the channel as free based on energy detection. The equipment has to observe the channel for a certain minimum during the CCA. The channel is considered occupied if the detected energy level exceeds a configured CCA threshold. If the channel is classified as free, the equipment is allowed to transmit immediately. The maximum transmit duration is thereby restricted in order to facilitate fair resource sharing with other devices operating on the same band.
Considering the different regulatory requirements, it is apparent that the LTE specification for operation on unlicensed bands will required several changes compared to the current Rel-12 specification that is limited to licensed band operation.
As already briefly discussed, in order receive or transmit data burst, a user equipment (UE) is synchronized to a serving cell. In the LTE system, this synchronization is achieved by the transmission of primary synchronization signals (PSS) and secondary synchronization signals (SSS). These signals are transmitted periodically with a fixed time pattern. This means that once an UE has knowledge of the PSS/SSS transmission pattern, it will know exactly when the next synchronization will be send. The periodic reception of synchronization signals is required for maintaining time, frequency and phase synchronization all the time. A detailed description of the PSS/SSS related procedures given in Section 7.2 of the book “LTE The UMTS Long Term Evolution: From theory to practice”, 2nd edition, ed. By S. Sesia, I. Toufik, M. Baker, Wiley, 2011.
Within the context of small cell enhancements is it currently discussed at 3GPP to support increased synchronization and discovery signal transmission intervals for the purpose of interference reduction and energy saving. This is described in the contribution TR 36.872 v12.1.0, “Small cell enhancements for E-UTRA and E-UTRAN—Physical layer aspects”, December 2013.
The procedures based on primary and secondary synchronization signals (PSS/SSS) have therefore been extended by the concept of configurable discovery reference signals (DRS). The DRS consists in general of a configured combination of PSS/SSS, (common reference symbols) CRS, (positioning reference symbols) PRS, and (channel state information reference symbols) CSI RS together with quasi co-location information (QCI) regarding the different reference symbols. The exact structure of supported DRS configurations is still under discussion at 3GPP, but the general assumption of fixed transmission intervals is still valid.
In the following discussion, synchronization and discovery on unlicensed bands is performed using PSS/SSS. The technical concepts assuming PSS/SSS for synchronization and discovery in unlicensed bands, are however not restricted to PSS/SSS as used by LTE on licensed carriers and the general technical concept can be applied for any DRS configuration (in terms of combinations of different reference signals) as well.
The focus of the invention is the LTE synchronization and discovery of LAA capable UEs in unlicensed bands under the assumption of strong unpredictable interference from other RATs, such as WiFi interference in the 5 GHz spectrum (overlapping cells).
A possible solution for LTE synchronization and discovery in unlicensed bands would be the use of Rel-8 PSS/SSS transmission patterns with a fixed 5 ms duty cycle.
FIG. 6 shows a typical case of collisions between Rel-8 PSS/SSS and a Wi-Fi transmission burst. Here it is assumed that the transmitting LTE node (small cell) performs CCA prior to transmitting a downlink (DL) burst, but not for PSS/SSS transmissions.
Depending on the node positions, the interference caused by the Wi-Fi burst to overlapping PSS/SSS transmissions can be quite severe, making the PSS/SSS undetectable or unusable for the UE intending to perform synchronization and cell discovery in the unlicensed band. The PSS/SSS that are transmitted within the LTE LAA DL burst are implicitly protected from Wi-Fi interference by the LBT procedure performed by the transmitting LTE node. The reason for this implicit protection is that the LTE node will only initiate a burst transmission after the channel has been detected as unoccupied. Since other equipment such as Wi-Fi nodes will perform LBT as well, they will not access the channel after the LTE node started a burst transmission which means that the PSS/SSS transmission within the LTE burst will be protected.
According to the above scheme, which makes use of a transmission pattern with a fixed duty cycle also for LTE in unlicensed bands, the UE can only rely on its own capabilities in terms of PSS/SSS detection even in case of strong interference from other RATs.
Such a scenario was however not foreseen during the Rel-8 PSS/SSS since the assumption was at that time operation in licensed bands with exclusive restriction to the operation of LTE by a single operator. The operator could here for example align the PSS/SSS transmissions of interfering cells in order to facilitate efficient interference cancellation in the UE. This is not possible in unlicensed bands since operators do not have exclusive control over the resource utilization in these bands, the transmission patterns of coexisting RATs (such as IEEE 802.11) are unpredictable and can not be controlled by the LTE operator.
Interference cancelation as a means for improving the PSS/SSS detection/decoding on the UE side would therefore be much more challenging than the corresponding interference cancellation under the assumption of exclusive LTE (PSS/SSS) interference. The unpredictable and uncontrollable Wi-Fi transmission can therefore result in quite long durations between PSS/SSS reception with sufficiently low SINR in order to maintain time and frequency synchronization at the UE all the time.
Another strategy, which envisages increased DRS duty cycles (e.g. 40 ms or 80 ms) is being currently discussed at 3GPP within the scope of small cell enhancements. This solution, however, does not solve the problem of the interference. On the contrary, according to this scheme the UE needs to wait significantly longer for the next DRS transmission than in case of Rel-8 PSS/SSS transmissions with 5 ms duty cycle. Therefore, the choice of increased DRS duty cycles will exacerbate the problem causing a severe synchronization loss in case of a strongly interfered DRS.
Another non-negligible critical issue regarding the transmission of PSS/SSS or any other kind of synchronization and discovery signal, such as DRS (discovery reference signals) in case of small cell enhancements, with fixed duty cycles is that the regulation for unlicensed bands at 5 GHz does in certain regions (such as for example Japan) not allow any kind of transmission without prior CCA performed by the transmitting equipment. Such regulation requirements are described in R1-144348, “Regulatory Requirements for Unlicensed Spectrum”, Alcatel-Lucent et al., RAN1#78bis, September 2014. As an example, if the regulatory requirement that is to be taken into account when designing LAA procedures is LBT, the PSS/SSS transmission interval may be increased. This approach corresponds to the DRS approach for small cell on/off with configurable transmission interval currently under discussion at 3GPP. A DRS comprises a combination of PSS/SSS and additional reference signals such as for example CRS (common reference symbols), CSI-RS (channel state information reference symbols) or PRS (positioning reference symbols).
However, also in this approach will not solve the problem due to unpredictable WiFi interference.